The rule caol le caol agus leathan le leathan means that the vowels on either side of a consonant (or group of consonants) should agree; they should both be broad or both be slender. The rule is primarily used when you add an ending to a word (e.g., when conjugating a verb). To satisfy the rule you may need to add a vowel between the word and its ending. Note that there are a few common words that do not satisfy this rule, as well as some compound words.

However, even without knowing a single word of Irish, you can apply the rule to catch many spelling mistakes! Let's try a few examples:

folús Focus on the consonant in the center, l, and look at the vowels immediately before and after. The vowel before, o, is broad, and so is the vowel after, ú. This word follows the rule, so there aren't any obvious spelling mistakes here. (In fact, it is spelled correctly.)

glacfidh Focus on the consonant group in the center, cf, and look at the vowels immediately before and after. The vowel before, a is broad, but the vowel after, i is slender. This looks like a mistake! (In fact, the correct spelling is glacfaidh).

tiocfaidh The flanking vowels, o and a are both broad, so this word follows the rule.

chruinneoinn The flanking vowels, i and e are both slender, so this word follows the rule.

dóimaid Uh oh! The vowel before, i, is slender, but the vowel after, a is broad. (In fact, the correct spelling is dóimid).

Now you are ready to find spelling mistakes in that letter from your Gaeilgeoir friend. This rule won't catch all spelling mistakes, but it will catch the most common ones.

Here are some words you can practice with. In each pair, one word is misspelled and one is spelled correctly. Can you pick out the correct one?